Integrally bladed rotors, in which the blades and disc form one continuous piece of metal, are used in gas turbine engines. Integrally bladed rotors used in compression systems are typically manufactured from titanium. Parts of the integrally bladed rotor require heat treatment following welding or the deposition of material in the fabrication or repair process. Typically an integrally bladed rotor made from titanium will require a heat treatment of the order of 1-2 hours in a temperature range of 500-700 degrees Celsius.
Heat treatment of the whole integrally bladed rotor, particularly after a repair, is not always necessary and can result in problems such as the formation of alpha-case on the finished titanium component.
It is known to locally heat parts of an integrally bladed rotor using induction or radiant heaters. U.S. Pat. No. 6,787,720 B2 discloses a device, which comprises a jacket made from a high temperature cloth material. Heating elements are woven into the fabric of the jacket and are used to heat treat part of an aerofoil in an integrally bladed rotor.
For radiant heating to be effective in delivering heat to a small repaired volume the heating elements typically have to be above the target temperature. This has the disadvantage that there is a significant risk of overheating thinner sections of the aerofoil, such as the leading and trailing edges.
Brazing is a joining process whereby generally a non-ferrous filler metal and an alloy are heated to melting temperature and distributed between two or more close-fitting parts by capillary action. Localised heat treatment of brazing systems risk damaging the workpiece and require considerable skill and complex control systems. Laser brazing uses a high intensity heat source that requires rapid movement to avoid local heat rises and surface over temperature, which can cause corrosion and fatigue. Brazing using an oxy-acetylene torch is diffuse heating, but the torch is oxidising and inappropriate for reactive materials and the gaussian-type heat spread complicates control of the gas temperature.
Components are often thermally cycled to test for thermal-mechanical fatigue, or observe crack growth. Samples are heated by furnaces or induction coils and subsequently removed to undergo cooling by air jets, fans, water or nitrogen. As the component must be removed from the furnace during each cycle, the cycle time is relatively poor and it is also difficult to regulate the temperature of the sample, especially where complex component geometries are employed. Heat also flows through the gripping arrangement. Induction heat treatment is highly sensitive to minor positional variations and geometric features such as notches or grooves, where the induction heater can fail to heat the internal faces of the notch, which can lead to a significant temperature gradient in the groove. Where the component can be heated through induction the induced magnetic fields can cause problems with instrumentation.
There can also be difficulties observing test results as oxidation of the test piece at high temperatures can obscure the sample and the formed coating can influence test results. Maintaining an inert shield for current heating methods is impractical.